Nonlinear signal filtering technology has been shown to be an effective method to limit the peak-to-minimum ratio of an envelope varying signal (for example as described in U.S. Pat. No. 7,054,385 issued May 30, 2006 entitled “Reduction of average-to-minimum power ratio in communications signals”, incorporated herein by reference), for signals with moderate peak-to-average power ratio. Under these conditions, this method provides for acceptable degradation in in-band performance measurements (e.g., error vector magnitude, or EVM, and rho, or the normalized cross-correlation coefficient between the transmitted signal and its ideal version) while the out of band performance (e.g., Adjacent Channel Power Ratio, or ACPR, Adjacent Channel Leakage Ratio, or ACLR, etc.) is not significantly degraded.
For signals that have large peak-to-minimum and large peak-to-average power ratios (e.g., UMTS downlink and Multi-carrier signals) extensions of the aforementioned nonlinear signal filtering technique can be applied. However, their effectiveness is limited due to the distortion they introduce to the signal. While extensions of these methods provide indiscernible degradation on the signal power spectral density (PSD), due to the distribution of the envelope of high peak to average power ratio signals the in-band signal fidelity (EVM) is significantly degraded. As such, there is a trade-off of in-band signal fidelity (EVM) with envelope minimum, where higher envelope minimum values cause higher distortion (i.e., higher EVM).
A similar problem may arise in the case of U.S. Pat. No. 5,727,026, “Method and apparatus for peak suppression using complex scaling values,” incorporated herein by reference, which addresses a distinctly different problem, namely reducing the Peak-to-Average power Ratio (PAR) of a communication signal. Large PAR is a problem for many, if not most, conventional power amplifiers (PA). A signal with a large PAR requires highly linear amplification, which in turn affects the power efficiency of the PA. Reduction is accomplished by adding a pulse to the original pulse-shaped waveform, with the pulse having an appropriate magnitude and phase such that the peak power is reduced. The pulse can be designed to have any desired spectral characteristics, so that the distortion can be kept in-band (to optimize the Adjacent Channel Power Ratio (ACPR)), or allowed to leak somewhat out-of-band (to optimize the error vector magnitude (EVM)). In any event, signal distortion is introduced. Not all signal types may be expected to tolerate such distortions.
The present invention addresses these signal degradation problems.